1. Field of the Invention
Aspects of the invention are directed to apparatus that detects arc faults from characteristic disturbances in load current while avoiding unnecessary trips resulting from disturbances in load current caused by other phenomena.
2. Background Information
Arc fault detectors for use in AC and DC electrical power systems typically monitor load current for random disturbances characteristic of such faults. However, there are many “normal” events that can cause disturbances in load current that must be distinguished from arcs to avoid nuisance trips. Some of these events are: the interruption of load current by a thermal switch such as an electric iron, a waffle iron or a furnace thermostat, the operation of a dimmer, especially when the setting is being changed, capacitor run motors, the startup of a compressor, and many others.
Many techniques have been proposed for distinguishing arcing from other load current events. U.S. Pat. No. 5,933,305 uses a cyclic current integration comparison in which the integrated value of sensed current for repetitive time intervals, typically one cycle for AC systems, is compared with the value of the previous interval. For each time interval, indications of interval to interval increases and decreases in the integrated values for a selected number, such as six, of the most recent time intervals, are counted. If a weighted, time attenuated accumulation of the counts reaches a predetermined amount, the arc detection signal is generated, which can be used, for instance, to trip a circuit breaker.
There is room for improvement in arc fault detection, especially in meeting the performance described in Underwriter Laboratories Standard UL1699 relating to clearing time, nuisance tripping, masking of arcing events and multiple load testing. There is also the challenge of simplifying the technical approach, which can potentially reduce the cost of arc fault detectors.